1. Field of the Invention
The present invention relates to a lead frame, and more specifically to a lead frame having wing leads.
2. Description of the Related Art
Referring to FIG. 1, there is shown one example of a conventional lead frame, which includes an island (or die pad) 4 for bearing a semiconductor chip (not shown), a pair of hanger pins 1 extending from opposite sides of the island 4 and supporting the island 4, a number of outer leads 2A extending to the outside of a plastic or resin packaged semiconductor device, a number of inner leads 2B located inside of the resin packaged semiconductor device and extending from a proximity of the island 4 to corresponding outer leads 2A, and a pair of tie-bars 6 for tying the outer leads together and for preventing flow-out of resin when resin packaging is carried out.
In a method of manufacturing a resin packaged semiconductor device using the lead frame mentioned above, a semiconductor chip (not shown) is die-bonded on the island of the lead frame mentioned above, and then, wire bonding is carried out to interconnect each of the electrodes on the semiconductor chip and a corresponding inner lead 2B, and thereafter, the assembly thus obtained is molded or packaged with a resin in a volume defined by a mold line 5 shown in FIG. 1. In this resin packaging process, the resin flowing out between each pair of adjacent leads is blocked by the tie-bars 6, so that the resin remains in the form of a burr or flash between each pair of adjacent leads on the inside of the tie-bars 6. Furthermore, the flash between each pair of adjacent leads and the tie-bars is cut out by a cutting die. Thereafter, the outer leads are plated with a desired material, and then tip end of each output lead and the hanger pins are cut, so that the resin packaged semiconductor device is separated from the lead frame. In addition, the outer leads are shaped into a desired shape. Thus, the resin packaged semiconductor device is completed.
When the resin packaged semiconductor device manufactured as mentioned above is mounted on a printed wiring board using solder, a problem is often encountered in that, because of thermal shock at the time of mounting the resin packaged semiconductor device by immersing it into a solder dip, moisture penetrates into the resin molded body and gasifies before it is expelled from the resin molded body, such that the gasified moisture expands within the resin molded body. As a result that, a crack occurs in the resin molded body.
In order to solve this problem, for example, Japanese Patent Application Laid-open Publication JP-A-63-133656 proposed a lead frame having a wing lead as shown in FIG. 2. The disclosure of JP-A-63-133656 is incorporated by reference in its entirety into the present application. In FIG. 2, elements corresponding to those shown in FIG. 1 are given the same Reference Numerals.
As seen from a comparison of FIGS. 1 and 2, the lead frame shown in FIG. 2 includes, in addition to the hanger pins 1, a pair of extensions 11 which extend from a pair of opposite sides of the island 4 orthogonal to another pair of sides from which the hanger pins 1 extend, each of the extensions 11 extending between a pair of inner leads 2B and further between a corresponding pair of outer leads 2A to reach the tie-bar 6 and to be coupled to the tie-bar 6. The extensions 11 support the island 4, but extend between the leads. Thus, in order to distinguish the extensions 11 from the hanger pins 1, the extensions 11 are called "wing lead," in this specification.
These wing leads function to expel the moisture which penetrates the resin molded body to the outside of the resin molded body, and therefore can prevent occurrence of a crack which would otherwise occur because of the heat added at the time of mounting the resin packaged semiconductor device on the printed wiring board. Therefore, the wing leads are provided at a location where the distance between the edge of the island and the edge of the resin molded body (namely, the mold line 5) is the shortest. For example, in the lead frame shown in FIG. 2, the wing leads are provided at the center in a longitudinal direction of the resin molded body. In addition, since the wing leads also acts as an island support lead, resin molding can be carried out reliably.
However, a portion of the extension 11 positioned at the outside of the resin molded body (the outside of the mold line 5) has to be cut out since that portion is not needed for the wing lead. Therefore, the extension 11 should be cut out together with the flash between each pair of adjacent leads and the tie-bars in a conventional tie-bar cutting process in which the flash between each pair of adjacent leads and the tie-bars are cut out by a cutting die. However, in order to cut the extension 11 together with the flash between each pair of adjacent leads and the tie-bars, since the extension 11 is added to the flash between each pair of adjacent leads and the tie-bars, it is necessary to modify a punch and a die of a cutting die set to a shape capable of simultaneously cutting out not only the flash between each pair of adjacent leads and the tie-bars, but also the extensions 11. This is expensive.
On the other hand, a remote tie-bar design is known as an alternative to the conventional tie-bar design as mentioned above.
In the remote tie-bar design, a tie-bar is located at the outside of a tip end of an outer leak, instead of locating the tie-bar at a base end portion of the outer lead in the conventional tie-bar design.
Referring to FIGS. 3A and 3B, there are shown diagrammatic partial side views of resin packaged semiconductor devices, after a lead shaping step, manufactured by using the conventional tie-bar lead frame and the remote tie-bar lead frame, respectively, for the purpose of illustrating a positional relation between the tie-bar and the outer lead in the conventional tie-bar design and the remote tie-bar design. In these Figures, Reference Numeral 8 designates a resin molded body encapsulating a semiconductor chip (not shown) therein.
As shown in FIG. 3A, in the conventional tie-bar design, the tie-bar 6 is provided on a base end portion of the outer lead 2A. On the other hand, in the remote tie-bar design, as shown in FIG. 3B, the tie-bar 6 is provided at the outside of a tip end of the outer lead 2A, namely, outside of a lead cutting position, so that when the outer leads are cut, the tie-bar is cut out together with a cut-out portion 12 of the outer leads.
As mentioned hereinbefore, when a lead frame based on the conventional tie-bar design as shown in FIG. 3A is used, after the resin encapsulating, the tie-bars and the flash between each pair of adjacent leads are cut by the cutting die and removed. On the other hand, when a lead frame based on the remote tie-bar design as shown in FIG. 3B is used, since the tie-bar is located outside of the tip end of the outer lead 2A, the resin will flow out to the outside of the tip end of the outer lead 2A at the time of resin molding. After the resin molding is completed, the outer leads are cut at the lead cutting position as shown in FIG. 3B. With this cutting, the tie-bars 6 are also cut out and removed. However, the flash remains between each pair of adjacent outer leads.
As seen from the above, since the tie-bars 6 are cut out when the outer leads 2A are cut, the tie-bar removing step, which was required in the conventional tie-bar design, becomes unnecessary.
Thereafter, the flash remaining between each pair of adjacent outer leads is removed by pressure jet honing (a high pressure water blowing) or the like (deflashing).
In the remote tie-bar design, accordingly, the cutting die modified to cut out the tie-bars and the flash remaining between each pair of adjacent leads, which was required in the conventional tie-bar design, is no longer necessary, since the remote tie-bar design can remove only the flash remaining between each pair of adjacent leads, by means of pressure jet honing (a high pressure water blowing) or the like.
Therefore, if the remote tie-bar design is adopted, it becomes unnecessary to prepare a separate punching die for each kind of semiconductor devices in the manufacturing process, and the tie-bar cutting step can be eliminated since the flash remaining between each pair of adjacent leads can be carried out at the same time as the step of removing a thin flash remaining on the leads, which is conventionally performed in the prior art.
This is easily seen from a comparison of FIGS. 3A and 3B. Furthermore, in the remote tie-bar design, after the outer leads are cut, since no part of the tie-bar remains in the outer leads, the lead shaping can be easily and reliably performed.
As seen front the above, if the remote tie-bar design is adopted, both the tie-bar cutting die and the tie-bar cutting step are unnecessary, and in addition, the lead shaping is reliable, and the desired lead shape can be easily obtained. However, for example, if it is desired to provide the wing lead mentioned hereinbefore in the remote tie-bar lead frame as shown in FIG. 4 (in which elements corresponding to those shown in FIG. 1 are given the same Reference Numerals), when the outer leads 2A are cut, the tie-bars 6 are simultaneously removed. However, there remain the wing leads having the same length as that of the outer leads. The flash between each pair of adjacent leads can be removed by pressure jet honing, but it is impossible to remove an unnecessary portion of the wing lead by pressure jet honing. As a result, it becomes necessary to prepare a wing lead cutting die, and to add a wing lead cutting step. Thus, the above mentioned advantage of the remote tie-bar design will be lost, to a great extent.
In addition, in the remote tie-bar lead frame shown in FIG. 4 having wing leads, the spacing between a wing lead 3 and an outer lead 2A adjacent to the same wing lead 3 is clearly narrower than that between a pair of adjacent outer leads 2A. Therefore, when the pressure jet honing is performed to deflash or to remove the flash remaining between each pair of adjacent leads, it is difficult to remove the flash remaining between the wing lead 3 and each adjacent outer lead 2A. If the pressure jet honing process is modified to be able to remove the flash remaining between the wing lead 3 and each adjacent outer lead 2A, the outer leads 2A are deformed by the pressure jet.
In addition, independently of-the remote tie-bar design, when the inner leads 2B are designed, in the case where the wing lead 3 is connected to the tie-bar 6, the position of the wing lead is determined by a wing lead cutting die, with the result that the degree of freedom in designing the inner leads 2B is restricted.